The present invention provides a method of identifying a base at a target position in a sample nucleic acid sequence, said method comprising: subjecting a primer hybridised to said sample nucleic acid immediately adjacent to the target position, to a polymerase primer extension reaction in the presence of a nucleotide, whereby the nucleotide will only become incorporated if it is complementary to the base in the target position, and determining whether or not said nucleotide is incorporated by detecting whether PPi is released, the identity of the target base being determined from the identity of any nucleotide incorporated, wherein, where said nucleotide comprises an adenine base, an .alpha.-thio triphosphate analogue of said nucleotide is used, ant the Rp isomer of said analogue and/or the degradation products of said analogue are eliminated from the polymerase reaction step.